Hepatocyte Growth Factor (HGF, the same abbreviation having also been used to define a completely different substance, i.e. hemopoiesis Growth factor) described by Nakamura et al., 1989, and by Hiyazawa et al., 1989), also known as Scatter Factor (Naldini et al., 1991a; Weidner et al., 1991), has the unique feature of combining mitogenic and motogenic activities on its target cells. HGF is mitogenic for hepatocytes (Michalopoulos, 1990) and other epithelial cells, such as kidney tubular epithelium, melanocytes and keratinocytes (Kan et al., 1991; Rubin et al., 1991; Halaban et al., 1992; Matsumoto et al., 1991). In these cells, HGF also promotes "scattering" (Stoker et al., 1987; Gherardi et al., 1989; Weidner et al., 1990, 1991; Naldini et al., 1991a) and matrix invasion (Weidner et al., 1990; Naldini et al., 1991a), and has chemotactic properties (Morimoto et al., 1991; Giordano et al., 1993). The factor stimulates extracellular matrix (ECM) degradation, by enhancing the synthesis of enzymes involved in ECM proteolysis (Pepper et al., 1992; Boccaccio et al., 1994). HGF acts as a morphogen in neuro-ectodermal development in vivo (Stern et al., 1990), and induces three-dimensional organization of epithelial cells in vitro (Montesano et al., 1991). The factor also promotes the progression of carcinoma cells toward malignant invasive phenotypes (Weidner et al., 1990).
The receptor for HGF is the tyrosine kinase encoded by the MET proto-oncogene (Naldini et al., 1991a, 1991b; Bottaro et al., 1991), a 190 kDa heterodimer of an extracellular 50 kDa .alpha. subunit, disulfide-linked to a transmembrane 145 kDa .beta. subunit (Park et al., 1987; Giordano et al., 1989a) Both subunits derive from glycosylation and proteolytic cleavage of a 170 kDa single chain precursor (Giordano et al., 1989b).
The HGF receptor is expressed in adult epithelial tissues, including liver, intestine and kidney (Prat et al., 1991a; Di Renzo et al., 1991). It has been reported to be expressed in early stages of development of epithelial organs (Sonnenberg et al., 1993), and it is often overexpressed in epithelial cancer (Prat et al., 1991a; Di Renzo et al., 1991). We have shown that the receptor is also expressed in endothelial cells and that HGF is a potent angiogenic factor both in vitro and in vivo (Bussolino et al., 1992; Grant et al., 1993). The HGF receptor is also known to be expressed in some populations of blood cells, such as macrophages, but the meaning of such an expression, which is barely detectable in the absence of activation, has not been elucidated (Galini et al., 1993).
EP-A-0,492,614 discloses the use of HGF as a growth enhancer for epitheliocytes, whereas WO 93/08821 discloses the use of HGF for the prevention of the side-effects of chemotherapeuticals.
Now it has been found that the hepatocytes growth factor induces proliferation and differentiation of multipotent and erythroid hematopoietic progenitors.
Hematopoietic cell growth and differentiation is under the control of a complex network of cytokines, which act on their target cells via specific receptors (Metcalf, 1984; Clark and Kamen, 1987). Erythropoiesis is a complex process in which a specific genetic program is primed (commitment) and executed (maturation). Although much is known about maturation, most of the molecular events occurring during the commitment phase are still obscure. Growth and differentiation of erythroid precursor are regulated by humoral factors and by largely uncharacterized cell-cell interactions with bone marrow stroma, the so-called hematopoietic microenvironment. Erythropoietin has long been considered the major factor required for erythropoiesis (Koury and Bondurant, 1990), other factors being far less specific (IL-3, GM-CSF, TGF-.beta.; Gasson, 1991; Miyajima et al., 1993; Sporn and Roberts, 1992). HGF represents a novel example of a humoral factor specifically active on erythropoiesis.
Recently, the synergism between HGF, IL-3 and GF, IL-3 and GM-CSF in promoting the growth of uncharacterized colonies from unfractionated murine bone marrow (Kmiecik et al., 1992) has been described. From said work, however, no conclusions about the effect of HGF alone could be deduced; moreover, the results obtained using bone marrow cell suspensions (including lymphocytes and monocytes-macrophages) could be ascribed to an indirect effect, mediated by the production of hematopoietic cytokines by accessory cells.
The synergism between HGF, IL-3 and GM-CSF, in fact, has not been confirmed by the authors of the present invention on isolated colonies of human hematopoietic cells.